Probably the worst advice a tradie could have told you. You need to get rid of all the rotted timber and either replace them or if your decking frame/structure bearers and joists are still ok you can put decking back over the top of the joists or you can place compressed fibro sheeting over the joist and then waterproof and tile it. First you will need a proper carpenter or builder to see if your bearers and joist can support the weight of the compressed fibro if you decide to go down that path.

Were you wanting to change to a tiled area as opposed to redoing the decking boards? I doubt it'd be cheaper to sheet and tile than lay new boards, particularly if, as mentioned above, you may need to reengineer your subfloor to handle the weight.

Had a tradie look at my deck today which has a decent amount of decayed timber on it and his advice was just to place cement sheeting over it and waterproof it

Im not familiar and was wondering what the consequences could be?

one thing on my mind is termite infestation unless the existing timbers were sealed up somehow.

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Problem will be is once you just cover up the old decking with sheets,the below decking will just fall apart and someone walking on that surface area will go through the sheet and decking,very dangerous,you will have to check from the pillars up the whole lot may need replacement..imho..

Find another Tradie! Someone that will check the condition of joists and bearers as well.
I have seen some shocking decks in WA - owner built jobs using untreated timber - plain steel nails and no idea on sizing and spacing on structural timbers. Not sure how high this deck is off the ground but there has been some horrible accidents in the past with collapses.
Well worth getting another opinion and getting it fixed right for safety as well as looks.

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